Ultrasonic flow meters typically have a pair of transducers which makes a direct contact with the process fluid. This type of flow meter configuration is referred to as “wetted” application. Each transducer is mounted on a transducer holder which is attached to a nozzle or port of a pipe or other flow channel (collectively referred to here as a “flow cell”). Flow meters transmit ultrasonic pulses into the flow cell and through the flow stream passing through the flow cell. The flow stream affects the transmitted pulses by, for example, altering the travel time of the pulses between a transmitter and receiver. By measuring the effect on the received pulses the flow meters can determine the flow rate of the stream.
Ultrasonic transit time flow meters generally have dual transducers that both emit and receive ultrasonic signal pulses. The dual transducers include an upstream transducer and a downstream transducer which are positioned such that the ultrasonic signal pulse or beam is at an inclined angle with respect to the axis of the flow cell. The upstream transducer emits an ultrasonic signal pulse that propagates through the flow cell and flow stream in a generally downstream direction. The signal pulse is received by the downstream transducer. In addition, the downstream transducer emits an ultrasonic pulse in an upstream direction that is received by the upstream transducer. The flow meter determines the difference in travel time between the ultrasonic pulses passing in a downstream direction and those passing in an upstream direction. The difference in pulse travel time can be used to determine the rate of the flow stream.
The transducer receives ultrasonic pulses and generates an electrical signal indicative of when the pulses are received. The transducer may convert to electrical signals only those pulses having a frequency similar to that of the transmitted pulse. The ultrasonic signals received by the receiving transducer includes the pulses that were originally emitted by the transmitting transducer. The received signals also include ultrasonic noise, so-called “acoustic short-circuit” noise, which consists of unwanted non-fluid-borne signals having a frequency within the range of frequency that the transducer detects. Short circuit noise typically arises from vibrations generated by the transmitting transducer and imparted or coupled mechanically to the transducer support and/or flowcell. These vibrations travel from the transmitting transducer by way of transducer shafts, transducer holders, transducer nozzles and the metal wall of the flow cell. These vibrations may have the same frequency as do the ultrasonic pulses that are transmitted through the fluid flow stream. However, these vibrations “short-circuit” the fluid flow by passing through the solid structures associated with the transducer and flow cell wall, and do not pass through the flow stream. Short circuit vibrations that are received by the transducer contribute to electrical signal noise in the measuring system. Short circuit noise interferes with achieving high accuracy and may obscure the data from the received signals from the ultrasonic pulses that pass through the flow stream.
To detect fluid flow through a pipe, ultrasonic flow meters typically use acoustic waves or vibrations having a frequency greater than 20 kHz (kilohertz). These flow meters preferably have a high SNR (signal-to-noise ratio) of greater than 20 decibels (dB). A high SNR promotes reliable signal detection, robust performance of the flow meter, and accurate readings of the flow rate by the flow meter. Conventional transit-time ultrasonic flow meters have attainted SNRs of greater than 20 dB for most liquid flow applications and in some high pressure gas flow applications. The SNRs tend to be low, e.g. less than 10 dB, for conventional transit-time ultrasonic flow meters measuring: low pressure gas flows, e.g. atmospheric flows; gases at high flow rates; high temperature gas flows that require transducers tolerant of high temperatures, and gas flows including saturated steam with condensed water. Accordingly, there is a long felt need for transit-time ultrasonic flow meters that have high SNRs when sensing gas flows having low pressures, high gas temperatures, high gas flow rates, and gas flows having saturated steam.